Change style of TextBlock for TargetNullValue - silverlight

Question

I want to change the Style of a TextBlock if the value of the bound property is null. I have specified a value for TargetNullValue of the TextBlock to be displayed, but i want to display it with an alternativ style. How can i do it.
My current solution is to use two TextBlocks and control the Visibility of both, to toggle between original and alternativ style. But this solution is not viable, cause i need to duplicate each TextBlock, for displaying alternativ version.
Current Solution:
<TextBlock Visibility="{Binding MyText, Converter={StaticResource nullToVisibilityConverter}}"
FontSize="20"
Foreground="Black"
Text="{Binding MyText}" />
<TextBlock Visibility="{Binding MyText, Converter={StaticResource nullToVisibilityConverter}}"
FontSize="20"
FontStyle="Italic"
Foreground="Gray"
Text="None" />
Needed Solution:
<TextBlock FontSize="20"
Foreground="Black"
Text="{Binding MyText, TargetNullValue='None'}" />
<!-- plus any styles, templates or triggers, to change style of TextBlock for TargetNullValue -->
How can i use an alternativ style for a TargetNullValue. Any solutions using Styles, Triggers or Templates are welcome.

Answer 1

Note: this is for WPF, you might have to convert anything that doesn't quite mesh with SL5.0.
The easiest solution (unless this requirement is ubiquitous) is to make a specific style for each instance which binds to the same property as the textblock, checks for Null and sets properties there.
This example will paste nicely into Kaxaml.
<Style x:Key="tacoStyle" TargetType="TextBlock">
<Style.Triggers>
<DataTrigger Binding="{Binding Source={StaticResource taco}}" Value="{x:Null}">
<Setter Property="Foreground" Value="Red"/>
</DataTrigger>
</Style.Triggers>
</Style>
</StackPanel.Resources>
<TextBlock Style="{StaticResource tacoStyle}" Text="{Binding Source={StaticResource taco}, TargetNullValue='bacon'}"/>
Of course if you need a more generic solution, you could extend TextBlock and add some logic to handle this.
<StackPanel>
<StackPanel.Resources>
<x:NullExtension x:Key="taco"/>
<Style x:Key="AltTacoStyle" TargetType="yourNS:ExtendedTextBlock">
<Setter Property="Foreground" Value="Pink"/>
</Style>
</StackPanel.Resources>
<yourNS:ExtendedTextBlock Text="{Binding Source={StaticResource taco}, TargetNullValue='bacon'}"
AltStyle="{StaticResource AltTacoStyle}"
AllowAltStyleOnNull="True"/>
</StackPanel>
And the control:
public class ExtendedTextBlock : TextBlock {
public static readonly DependencyProperty AllowAltStyleOnNullProperty =
DependencyProperty.Register("AllowAltStyleOnNull", typeof (bool), typeof (ExtendedTextBlock), new PropertyMetadata(default(bool)));
public bool AllowAltStyleOnNull {
get { return (bool) GetValue(AllowAltStyleOnNullProperty); }
set { SetValue(AllowAltStyleOnNullProperty, value); }
}
public static readonly DependencyProperty AltStyleProperty =
DependencyProperty.Register("AltStyle", typeof (Style), typeof (ExtendedTextBlock), new PropertyMetadata(default(Style)));
public Style AltStyle {
get { return (Style) GetValue(AltStyleProperty); }
set { SetValue(AltStyleProperty, value); }
}
static ExtendedTextBlock() {
TextProperty.OverrideMetadata(typeof(ExtendedTextBlock), new FrameworkPropertyMetadata(default(string), PropertyChangedCallback));
}
private static void PropertyChangedCallback(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs) {
var tb = (ExtendedTextBlock)dependencyObject;
var binding = tb.GetBindingExpression(TextProperty);
if (binding != null && binding.DataItem == null) {
if (tb.AllowAltStyleOnNull)
tb.Style = tb.AltStyle;
}
}
}
You'll need to flesh that out a bit to be production-ready, but you get the idea.